11N USES 20-MHZ AND 40-MHZ CHANNELS. THE 40-MHZ CHANNELS IN 11...

802.11n uses 20-MHz and 40-MHz channels. The 40-MHz channels in 802.11n are actu-

ally two 20-MHz channels that are adjacent to each other and bonded.

Clients in 802.11n environments are pretty complex, so 802.11n is combined with OFDM.

This enables the use of more subcarriers that range from 48 to 52.

With 802.11n, you can get up to 32 data rates.

Sending Frames

For the allocated time to send frames, only CTS to self is used with 802.11n; the RTS/CTS

that was discussed earlier in this chapter is not used.

Another feature of 802.11n that makes it much more efficient is the way it uses block ac-

knowledgments as opposed to acknowledging each unicast packet like the other 802.11

protocols do. A block acknowledgment works by sending a number of frames before hav-

ing them acknowledged. This is similar to the way TCP works.

Another aspect of sending requires knowledge of how frames are sent in a normal 802.11

a/b/g world. You will learn more about this in Chapter 7, “Wireless Traffic Flow and AP

Discovery,” but the following is a quick look:

Each sending station must wait until a frame is sent before sending the next frame;

this is called distributed interframe space (DIFS).

This DIFS can cause more overhead than necessary. 802.11n improves on this DIFS mecha-

nism by using a smaller interframe space called reduced interframe space (RIFS). This re-

duces delay and overhead.

Antenna Considerations

The number of antennas that the sender and the receiver have can differ. Here is how

they work.

If a transmitter can emit over three antennas, it has three data streams. If it can receive

over three antennas, it has three receive chains. In documentation, this is called a 3 × 3. Two

receive chains and two data streams is called a 2 × 2.

This is important because the Cisco 1250 AP is a 2 × 3 device. If you have a laptop that is a

2 × 2, you can start to see how this takes on meaning. When using special multiplexing,

you are limited to the same number of streams as the lowest number of antennas. In this

scenario, you would have two streams.

Finally, note that even if you do not have 802.11n clients, you can expect to see about a 30

percent improvement, based on these features.

Exam Preparation Tasks

Review All Key Concepts

Review the most important topics from this chapter, noted with the Key Topics icon in the

outer margin of the page. Table 6-9 lists a reference of these key topics and the page num-

ber where you can find each one.

Table 6-9 Key Topics for Chapter 6

Key Topic Item Description Page Number

Table 6-2 The 802.11 protocol 100

Table 6-3 The 802.11b protocol 101

Table 6-4 The 802.11g protocol 102

Table 6-5 The 802.11a protocol 106

Table 6-6 The UNII frequency bands 107

Complete the Tables and Lists from Memory

Print a copy of Appendix B, “Memory Tables” (found on the CD) or at least the section for

this chapter, and complete the tables and lists from memory. Appendix C, “Memory Tables

Answer Key,” also on the CD, includes completed tables and lists to check your work.

Definition of Key Terms

Define the following key terms from this chapter, and check your answers in the Glossary:

FHSS, DSSS, ISM, OFDM, beacons, ERP, RTS/CTS, CTS to self, DFC, TPC, MIMO,

precoding, transmit beamforming, spatial multiplexing, channel state information,

block acknowledgments, DIFS, RIFS

End Notes

CWNA Certified Wireless Network Administrator; Official Study Guide, Planet 3

Wireless, McGraw Hill/Osborne 2005